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Actually, you can't merely determine the projection of the raft beyond the retaining walls. There are some interfering variable should be considered. In order to engineer the raft projection, firstly determine the water/proofing system if it will be block wall adjoined to the wall of another system and what is the thickness of that block work wall. Secondly, the loading condition of the RC walls relatively with the adjoined columns, you should investigate the soil pressure beneath this particular area. And finally, the property line.
the answer is D 30 to 45 CM
There are so many factor involved for the projection of slab beyond the column support / wall, it mainly depend upon the safe bearing capacity of soil and need to keep in mind that base pressure should not become negative, so to counter the negative/positive base pressure required projection need to be provided.
The projection (Toe) of raft depend upon base width of foundation and which is determined according to hight of retaining wall properties of retained structure. The projection should be 025 to 0.33 of base width. but should not be less than 30 cm.
Excuse me but this is a very unprofessional phrased/naive question. Anyone aware of the design process of a raft wouldn't dare to ask this question. Based upon design (bearing capacity, uplift, load concentration, ...etc)/property line, the answer ranges so widely.
The projection of Raft beyond the Outer face of the Structure depends on different design aspects -
- Buoyancy
- Additional wall around the structure for better water proofing
- Reinforcement Anchorage of main structure in the Raft
- Providing Haunch at the junction of Structure and Raft which will enhance joint durability
The length of projection may preferably range from 40 cm to 75 cm
